Riddle me this?!?! 3GS new boot rom with 6.15.00?!?

Ok so I bought an unlocked jail broken phone eBay a recent time ago. Unlocked for tmobile. It was running software version 4.1 with the iPad baseband 6.15.00 now heres the the confusing part I know for a fact this phone has the toshiba chipset and the serial number states it was made in the 49th week of 2011?!? How is this possible? To my knowledge this phone should have been perminately bricked right? I would post a picture but I'm a n00b to this site and don't know how any help here? Because I accidentally update the version to 5.1.1 and I had to switch to an at&t prepaid sim just so I could use my phone. Still under a t mobile contract and want to go back to tmobile. Please help a broths out!

well the phon isnt bricked
so to my (limited) knowledge of unlocking you are going to need to jauilbreak using redsnow and preserve baseband, and trhen gut ultrasnow and ultrasnow fixer and all that jazz, then your phone will be unlocked and you can use it on tmobil